The invention relates to a device and method for monitoring a telephone number of a user. A software radio platform and a global system of mobile communication (GSM) protocol stack program are utilized by the device and method to build a regulation base station system which has a base transceiver station (BTS) function of a GSM network, and the regulation base station system is used for conducting wireless communication with a telephone of the user and extracting identification information, like an international mobile subscriber identification number (IMSI), of the telephone of the user. According to the IMSI of the telephone of the user, a subscriber identity module (SIM) card simulation program is used for achieving SIM card simulation on the telephone of the user, the SIM card simulation program completes operations, such as network authentication and position updating, and then is attached to a normal network through information interaction between identity simulation equipment and a simulation user telephone SIM card, finally the telephone number of the user is extracted by dialing of a called terminal, and monitoring of the telephone number of the user is achieved according to a preset security policy. The device and method is mainly composed of six components including a software radio module, a called terminal module, an identity simulation module, an SIM card simulation module, a main control module and a GSM protocol stack analysis module.